Summon kidney stone.

Or the opponent has untreated ADHD causing them to keep losing their journal/spellbook, and, after searching their house, cleaning it, fixing a drawer slide, and picking up the lute for the fifth time in two years, they have finally found 4/5 of their lvl1 spellbooks.

All of the spells contained therein, however, are in various states of completion and riddled with attention-related errors, so, they may be able to put together a magic missile spell but it is quite likely to have unintended consequences like a half-eaten bagel materializing in the target’s pocket and spectators suddenly having minty-fresh breath.

The sigil used for casting is just the cool S everyone drew in highschool.

Every spell involves “your mom”.

I can only think of two plausible interpretations of this concept.

1. Mage is broke as hell and cobbled together his own grimoire by hand-copying spells from the college library, friends, and the occasional dungeon find. Yeah, a few occult incantations slipped in there, but you’d hardly notice for all the doodles and random “todo” lists from years ago.

2. Death note.

Either way

I cast liquid lungs

Power word: scrunch

I cast Tasha’s uncontrollable farts.

You become incapacitated and you emanate a poison cloud area that damage foes and friends in a 5ft radius.

Creatures with a fart fetish get advantage on attacks towards you.

Any creature in range can use their bonus action to jar your farts and sell them on the Fart market for 2d4 silver coins each.

Casts magic missle